In a wavelength division multiplexing optical communications system, information is carried by multiple channels each with a distinct wavelength range. It is often necessary to separate, combine, selectively attenuate or amplify each wavelength channel. In order to perform these functions one usually needs to separate the wavelength channels from one another. In this specification, these individual information-carrying lights of a wavelength division multiplexing optical fiber, optical line or optical system are referred to as either “signals” or “channels.” The totality of multiple combined signals, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.” Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted λ1-λn.
A de-multiplexer is an apparatus that receives a composite optical signal comprising a plurality of wavelengths or channels and separates the channels among a plurality of respective outputs. A multiplexer is an apparatus that receives a plurality of wavelengths or channels from separate respective inputs and combines them into a single composite optical signal directed to a single output. Because light paths are generally reversible through most optical components and apparatuses, such separation or combination can generally be performed by a single apparatus, depending upon the direction of light through the apparatus. Such an apparatus that can be utilized as either a multiplexer or a de-multiplexer is herein termed a multiplexer/de-multiplexer (MUX/DEMUX).
A diffraction grating is an effective wavelength dispersive component that can be used to either separate or combine wavelength channels. It is known that the resolving power of a diffraction grating not only depends upon the angular dispersion of the grating, but also upon the size of the optical beam incident upon the grating. The resolving power of a grating can be written as:       λ          Δ      ⁢                           ⁢      λ        =  mNwhere λ is the center wavelength, Δλ is the minimum wavelength difference that can be resolved, m is the diffraction order and N is the number of “grooves” illuminated by the optical beam. Apparently, because N is proportional to the width of the incident optical beam, the resolving power is linearly proportional to the width of the incident beam.
It is also necessary for the divergence of an optical beam incident upon the grating to be smaller then the angular dispersion provided by the grating to effectively separate two adjacent wavelength channels. If a lens is used to focus the diffracted beams to different respective spots, the resolving power expression also means that the diffracted beam of a wavelength channel must be focused into a spot that is smaller than the spatial separation of two adjacent wavelength channels at the focal plane.
Fiber collimators are often used to collimate divergent optical beams emerging from optical fibers. Because an optical beam emitted from a single-mode fiber is circular, the collimated beam comprises a round cross section. The diameter of the collimated beam is proportional to the focal length of the collimating lens. To achieve large beam size so as to illuminate an adequate number of grating grooves, one can choose large focal length collimating lenses. However, if this is done, subsequent large optical components must be provided to handle the resulting large beams. This usually creates an increase in system complexity and overall size.
It is realized by the inventors of the present invention that, for most fiber optics applications, the incident beam only needs to have a large width in the dispersive direction of the grating—that is, perpendicular to the grating “grooves” or other diffraction-causing pattern on the grating. This means that an anamorphic optical beam with an elliptical cross section can be effectively utilized to achieve high spectral resolution while preserving compact system size and simplicity. There is therefore a need, in the art, for a grating-based MUX/DEMUX that can utilize anamorphic optical and beams to minimize device size while maintaining adequate resolving power. The present invention addresses such a need.